In general, compressors are machines that are supplied power from a power generator such as electric motor, turbine or the like and apply compressive work to a working fluid, such as air or refrigerant to elevate the pressure of the working fluid. Such compressors are widely used in a variety of applications, from electric home appliances such as air conditioners, refrigerators and the like to industrial plants.
The compressors are classified into two types according to their compressing methods: a positive displacement compressor, and a dynamic compressor (a turbo compressor).
The positive displacement compressor is widely used in industry fields and configured to increase pressure by reducing its volume. The positive displacement compressors can be further classified into a reciprocating compressor and a rotary compressor.
The reciprocating compressor is configured to compress the working fluid using a piston that linearly reciprocates in a cylinder. The reciprocating compressor has an advantage of providing high compression efficiency with a simple structure. However, the reciprocation compressor has a limitation in increasing its rotational speed due to the inertia of the piston and a disadvantage in that a considerable vibration occurs due to the inertial force.
The rotary compressor is configured to compress working fluid using a roller eccentrically revolving along an inner circumference of the cylinder, and has an advantage of obtaining high compression efficiency at a low speed compared with the reciprocating compressor, thereby reducing noise and vibration.
However, in spite of the aforementioned advantages, the rotary compressor has a structural limitation not allowing the roller to revolve in both directions. In other words, the conventional rotary compressor is provided with only a single suction port and a single discharge port, which communicate with the cylinder. The roller performs its rolling motion from an inlet side to an outlet side along the inner circumference of the cylinder to compress the working fluid, such as refrigerant. Accordingly, when the roller performs its rolling motion in a reverse direction, i.e., from the outlet side to the inlet side, it is impossible to compress the working fluid.
Furthermore, the aforementioned structure of the conventional compressor makes it impossible to vary its compression capacity. Recently, there are appearing compressors in which the compression capacity is variably changed so as to correspond to a variety of operational conditions of air conditions. However, the conventional rotary compressor has a limitation in its application since it has only a single compression capacity.